1. Field of the Invention
This invention generally relates to a system for controlling the operation of a photoelectric element and particularly to a control system for the operation of an array of photoelectric elements, such as a line sensor for use in optical reading of an original image. More specifically, the present invention is concerned with a photoelectric element control system of the charge storage type in which a relatively large signal is obtained by storing the charge produced by irradiation of light over a predetermined time period.
2. Description of the Prior Art
In facsimile machines, optical readers and the like, use is made of an image sensor having a plurality of photoelectric elements arranged linearly or in a two-dimensional matrix form for reading an original image thereby converting optical image information into electrical image information. In such a photoelectric array, in order to minimize the number of connection lines to simplify the structure of its control circuit, the connection lines for respective photoelectric elements are divided into groups and the connection lines in the same group are commonly connected. With such a structure, a predetermined voltage is sequentially applied to the groups of photoelectric elements one after another to carry out the so-called matrix control. In addition, since the signal reading time period per pixel or dot is relatively short in this type of image sensor, the time period other than the signal reading time period is used for storage of charge produced by irradiation of light. It is often so structured that a predetermined bias voltage is applied to the photoelectric element to have it reverse-biased during the charge storage time period.
The typical prior art image sensor including a plurality of linearly arranged photoelectric elements is illustrated in FIG. 1a. As shown, the image sensor includes a plurality of photoelectric elements 111, 112, 13, . . . , 11n, 121, 122, . . . , 12n, 131, 132, . . . , 1mn, which are arranged linearly and divided into m number of groups, each group including n number of photoelectric elements whose anodes are commonly connected to the corresponding row line. Thus, there are m number of row lines. The image sensor also includes a like plurality of blocking diodes 211, 212, 213, . . . , 21n, 221, 222, 22n, 231, 232, . . . , 2mn, which are connected in series but opposite in direction to the corresponding photoelectric elements, respectively. Every n number of blocking diodes have their anodes commonly connected to the corresponding column line, and, thus, each column line is connected to m number of blocking diodes, one in each of n number of groups. It is thus clear that there are n number of column lines, to which are connected n number of switching elements S1 for controlling the application of read out voltage Vt and n number of switching elements S2 for controlling the application of ground potential.
Also provided as connected to the row lines are m number of switching elements S3 for controlling the application of charge storing voltage Vb and m number of switching elements S4 for selecting the row lines for operation. The switching elements S4 have their output terminals commonly connected to input terminals of a signal outputting switching element S5 and of a grounding switching element S6. And an amplifier AMP is provided with its input terminal connected to the output terminal of switching element S5. Each of the switching elements S1 through S6 is turned on when a control signal supplied is Hi and turned off when the control signal is Lo. As is commonly practiced, a character with overline indicates the complementary state of the binary state of the corresponding character without overline.
Now, the operation of the structure shown in FIG. 1a will be described also referring to FIG. 1b. If no signal reading takes place, the condition is established with all of the switches S1 off, all of the switches S2 on, all of the switches S3 on, all of the switches S4 off, the switch S5 off and the switch S6 on. Accordingly, the blocking diodes 2ab, where a ranges between 1 and n and b ranges between 1 and n, have their anodes grounded and voltage Vb is applied to the anodes of photoelectric elements 1ab. Under the condition, charge is accumulated in each of photoelectric cells in accordance with the amount of light irradiation.
If the signal reading operation is to be carried out sequentially starting from the photoelectric cell located at first row and first column, voltage Vb is first removed from the first row line and this row line is selected (i.e., .phi.x1 being Hi) as an output line. Then, after elapsing a predetermined time period, the voltage Vt is sequentially applied (i.e., .phi.yb being Hi) to each of the column lines at a predetermined friquency, whereby the output end of each of the row lines is connected to the amplifier AMP (i.e., .phi.xr being Hi) at that frequency. Upon completion of signal reading for the first row, voltage Vb is again applied to the first row line and at the same time voltage Vb is removed from the second row line so that the second row line is now selected as an output line. Then, similarly with the case for the first row, voltage Vt is sequentially applied to each of the column lines.
While signal reading is carried out in this manner, charge is accumulated in the parasitic capacitor present in each of the photoelectric cells owing to voltage Vb applied thereto during charge accumulation time period. The charge thus accumulated in the parasitic capacitor is discharged as soon as the photoelectric element is set in a signal reading mode. Thus, if signal reading is carried out immediately after setting the photoelectric element in a signal reading mode, the current associated with the parasitic capacitor comes to be mingled with an image signal read as a noise. For this reason, it is common practice to carry out an actual signal reading operation after elapsing of a sufficient time for the noise component to be discharged upon setting of a signal reading mode for photoelectric element. However, the longer this wait time, the slower the image reading speed; on the contrary, the shorter this wait time, the more the noise component in the signal read.
In general, since a relatively small current (dark or noise current) flows through the photoelectric element even without light irradiation, a reading error will result if one tries to associate the output signal directly to light intensity. Such a dark current varies depending upon the surrounding temperature or the like so that the signal level cannot be accurately compensated for even if various current components are simply subjected to addition or subtraction.